Log-shipping data replication is a common technique used by database systems to increase availability of the respective database to applications. A primary instance of a database transfers copies of its log records to a standby instance of the database, where the logged operations are replayed in a fashion similar to that of ordinary database recovery operations (e.g., crash recovery or rollforward recovery). The standby instance of the database is typically unavailable for update during normal operation, but can take over as a new primary instance of the database in case of a failure of the original primary instance of the database. The database is generally available as long as either site is functioning properly, providing protection against a single point of failure.
Two important measures of the usefulness of log-shipping data replication are transactional consistency and the performance impact on the primary instance of the database. Transactional consistency concerns the degree of guarantees the system provides that transactions performed at the primary instance of the database are neither lost nor completed with different outcomes at each standby instance of the database, both during normal operation and in case of the failure of either database instance. The performance impact on the primary instance of the database concerns the degree to which the normal processing of the primary instance of the database is slowed down in order to accomplish the log shipping data replication activity. These two characteristics can be tuned by configuration choices and other factors, and in some cases, the choices trade off one against the other. A challenge therefore for the computer industry is to optimize both characteristics to the greatest extent possible.